Last year, I approached and befriended a dude who I admired at my gym. I shit you not, his real name is shockingly similar to Chad. He stands about 6'2", keeps a blonde buzzcut, and looks like the Incredible Hulk's aesthetic cousin.
[Chaz] is efficient as fuck in the gym. He is in and out in under 30 minutes. That's it. He is always focused and I'm envious as fuck of his body. [Chaz] is a nice dude who was more than happy to share what he outs into his mixer (BCAAs and L-Glutamine) and his basic training regimen.
His number one secret, he says, is to work abs every single time you go into the gym, every day if you can. He does six sets of 15 decline sit ups 5 days a week, just adding them in at the end of his other sets. That doesn't take as long as you'd think.
I did some research and found out that the Terminator himself followed a similar ab routine: "I believe in doing some abdominal exercises every training day--or at least 6 times a week. When I was really in a hurry to get my midsection in shape, I worked abs twice a day." - Arnold.
Research further and you will find that the seven-time Mr. Olympia recommends 100-200 a sit ups a day. As all us normies know, moderation still yields results. As any good gym bros know, small muscle groups take way less time to recover. This includes your abs.
Ab work gets a bad rep in a lot of modern fitness circles, with a lot of lifters saying that they get worked by proxy in most exercises. This isn't wrong, but why settle for less?
If anything, we should consider that stronger abs means better lifts all around, since they are activated in almost every major exercise. They are the home to your center of gravity, your natural weight belt, and what every girl wants to look at before she closes her eyes to swallow a meat stick.
I've been running 3 sets of [Chaz's] 15 decline sit ups and a few posted leg lifts every time I step into the gym for the past five months and the results are undeniable. I've had a good six pack for awhile, but this advice kicked that shit into Kratos mode. Most noticeably, I have deeper definition, taller rectus abdomini, leaner love handles, and more visible serrati. It also gets my heart racing, so I often count this as my post-workout cardio.
Find the ab exercises that work for you, but remember some basic principles of biomechanics:
EDIT: To address a lot of comments...this end of workout ab burndown regimen is not a standalone workout routine. It's not for fatties and beginners. You should have dieting under control, be lean, and be consistent with weighttraining fundamentals before worrying about this. If you aren't concerned with targeting this muscles group, more power to you. Some people are, though, and don't consider that abs need to be hit on the regular to get next level results.
For the guys complaining that not all like abs, or that they also like other muscles...yeah. But still go out and improve your ab aesthetics if you want to. I doubt it will hurt your self esteem or rapport with women.